Radios generally function to receive broadcast signals. Usually, the user of a radio will not be personally interested in all of the signals that are broadcast on a communication resource (such as a particular frequency or a particular TDM time slot). For example, many broadcast signals include an identifying signal to facilitate proper processing in certain radios. Other broadcast signals include special identifying signals, such as tone signals or subaudible digital signals, that the radio can recognize and respond to.
Dispatch communication systems are known. In such a system, a user will typically not himself enter a specific code to identify a specific user. Instead, the user's communication unit has predetermined talk groups programmed into it to automatically provide the system with the appropriate information to allow establishment of a dispatch call. Therefore, the user often simply keys the microphone and begins talking without the need for any dialing or manual user selection mechanisms at all. The intended listener, being a member of the defined talk group (and all other members of the same talk group), will generally receive this voice communication, and the communication path establishment and maintenance protocol is substantially transparent to the users.
Many signals of interest, such as a talk group call, are prefaced with a specific identifying preamble, such as an ID (identification) for the transmitting radio. Preambles may be lengthy and are always at the risk of truncation, especially in systems employing scanning techniques or when multipath distortion or fading occur. Many systems cannot properly place a radio call without the preamble, resulting in lost transmissions. Clearly, this is not a desirable state for a communications system, especially when calls are urgent in nature.
Some systems with emergency capability rely on repetitive transmissions of short signals, such as a tone. The transmissions are spaced in time to enable an acknowledgment to be received by the transmitting radio. Typically, these transmissions contain no indication of the nature of the emergency. Some systems follow up acknowledgment of the emergency transmissions by remotely keying up the radio for a limited time, say 10 seconds, to monitor the situation. These remote key-ups may be delayed by several seconds, which could easily be critical seconds in an emergency.
Furthermore, it is essential that an emergency transmission be received by the dispatcher with a minimum of distortion or error in the transmission. Often, users turn down the transmit power output of a portable radio to conserve battery power. This practice may cause a transmission to be distorted sufficiently to render the emergency message useless in handling the situation.
Accordingly, a communication protocol which provides required system functions, including emergency operation, that are transparent to the user while minimizing loss of transmissions is required.